Generator/motor system and method for operating said system

ABSTRACT

The invention relates to a generator/motor system and a method for operating this generator/motor system with which the filter currents are reduced. The generator/motor system has, for this purpose, a rotational field machine (DM) and a pulse-controlled inverter and filter capacitors (C 1 , C 2 ). The pulse-controlled inverter is formed in the embodiment according to the invention by means of two identical pulse-controlled inverters (PWR 1 , PWR 2 ) which each have half the rated power. During operation, depending on the necessary rotational speed switching over is performed between a star circuit in which only the first pulse-controlled inverter is operational, and a single phase circuit in which both pulse-controlled inverters are operational. In order to obtain a torque which is comparable to the prior art, even if only one of the two pulse-controlled inverters is used, the rotational field machine has approximately twice the number of stator turns.

The invention relates to a generator/motor system as claimed in thepreamble of claim 1, and to a method for operating this generator/motorsystem.

At present, efforts are being made in motor vehicles with an internalcombustion engine to combine the starter and generator to form a singleelectric machine.

However, during these efforts there is a problem, even at the generaldesign stage, that two completely contradictory requirements have to bemet.

On the one hand, in order to start and speed up an internal combustionengine it is necessary to apply an extremely high turning torque. Thistorque may be, depending on the engine capacity or cylinder number ofthe internal combustion engine, greater than 240 Nm. Furthermore, theelectric machine must also be able to provide torque reserves forspeeding up the internal combustion engine to the starting speed.

On the other hand, after the internal combustion engine has beensuccessfully started, the electric machine which is designed as astarter/generator should operate predominantly as a generator in orderto feed into the on-board power system of the motor vehicle. In thiscontext, there is a need for a constant output of power over theextremely spread out rotational speed range, predefined by the internalcombustion engine, from 600 to 6000 1/min (motor) with the highestpossible efficiency.

It is virtually impossible to meet both requirements economically with astandard drive composed of a three-phase rotational field machine 30 andvoltage-impressing pulse-controlled inverter (PRW) 31 in a rotationalfield bridge circuit with filter capacitor C, as shown in FIG. 3A.

A problem which it is necessary to overcome in this context is thenecessary miniaturization and complete integration of the powerelectronics. The necessary filter capacitors are an impediment tointegration. In particular, given the relatively low on-board powersystem voltage of 42 V, phase currents of approximately 1200 A inasynchronous machines are currently under discussion in order togenerate the starting torques which are required. The intermediatecircuit capacitors C, such as are shown, for example, in FIG. 3A whichshows the design of a conventional drive system with rotational fieldmachine 30, pulse-controlled inverter 31 and intermediate circuitcapacitor C, assume considerable dimensions in this context, and thesedimensions are an impediment to integration.

Furthermore, first measurements in the absorber space have shown that itis not possible to make any compromises here. The filtering is necessaryin order to fulfill the stringent EMC requirements in motor vehicles. Itis imperative to reduce the currents of the machine and thus the filtercurrents while keeping the other properties of the drive the same.

The configuration of the drive system with a rotational field machineand pulse-controlled inverter is conventionally as follows, describedwith reference to FIG. 3B.

FIG. 3B shows a conventional rotational speed/torque characteristic. Thecontinuous line in FIG. 3B shows what can be achieved with a specificconfiguration of the rotational field machine and an associatedpulse-controlled inverter power.

If, for example, the starting torque is to be increased while retainingthe standard pulse-controlled inverter topology, i.e. onepulse-controlled inverter in a six pulse bridge circuit, and retainingthe pulse-controlled inverter (apparent) power, the winding of therotational field machine must be correspondingly changed. In thesimplest case, more turns with thinner wires are formed. This leads tothe characteristic curve shown by dashed lines in FIG. 3B. It isapparent that although this measure can increase the starting torquewith an unchanged pulse-controlled inverter power, this can only beachieved at the cost of the generator power at relatively highrotational speeds. The configuration point drops correspondingly. Owingto the relatively high number of turns, the rotational field machinereaches its field weakening mode, i.e. the modulation limit of thepulse-controlled inverter, earlier and is able to output less powerlater during the generator mode.

In particular in motor vehicle applications, and specificallystarter/generator arrangements, the costs for the pulse-controlledinverter also play a decisive role. The costs of a pulse-controlledinverter are nowadays no longer assessed very much according to thecurrent strength which the pulse-controlled inverter has to bear butrather according to the current strength which has to be commutated inthe topology. This characteristic variable determines the filterexpenditure which has to be made particularly in the especiallyEMC-sensitive field of the car industry. In addition, the filters are animpediment to miniaturization, as are in particular also the reliabilityproblems at high temperatures. For this reason it is necessary toattempt to configure the power electronics in the drive circuit asefficiently as possible, in particular to reduce the currents to becommutated.

M. Osama, T. A. Lipo “Modeling and analysis of a wide-speed-rangeinduction motor drive based on electronic pole changing”, IEEETransactions on Industry Application, Vol. 33, No. 5, September/October1997 describes a rotational field machine with poles which can beswitched over, two winding systems and two separate pulse-controlledinverters. However, less than optimum winding factors are obtained withthe specific combination of the winding systems so that the rotationalfield machine cannot convert the maximum possible pulse-controlledinverter current for a given overall size of the pulse-controlledinverter into the torque in an optimum way. The dynamic behavior whenthe rotational field machine is switched over is not possible withoutcorresponding torque transient effects, which can throw up particularproblems in the drive phase, which can adversely affect the user'scomfort. The Dahlander circuit which has also been known for a long timehas a similar problem.

In DE 199 31 010 A1, a so-called diode-clamp double-three-levelconverter, which is known per se, is actuated by a novel pulse method insuch a way that a parallel/serial switchover of the two winding systemscan be brought about. At the same time, the number of poles of therotational field machine can be retained during switching over. Sincethe switching over is brought about by a different predefinition of thevoltage vectors, the switching over also takes place with little noiseand without torque transient effects. In addition, the winding systemscan also be “pivoted” in their phase so that a further significantreduction in the intermediate circuit current to be filtered can bebrought about. Although this system is most developed technically, it isvery complicated and costly.

For this reason, a system with a feed power converter and a machinepower converter, that is to say a genuine converter, would be moresuitable since then a higher degree of flexibility can be achieved. Sucha genuine converter is described, for example, in L. Sack, “Reduction oflosses in the DC-link capacitor of two-stage self-commutatedconverters”, Proceedings of the EPE '99, Lausanne, Switzerland. In saiddocument it is also possible to achieve a significant reduction in theripple current to be filtered by synchronizing the pulse patterns. If itis possible to reduce the ripple currents, the efficiency of the systemcan also be increased simultaneously since a relatively large amount ofenergy at the feed power inverter of the capacitor is conventionallyalso converted into dissipated energy.

For this reason, the object of the present invention is to construct agenerator/motor system and a method for operating this motor/generatorsystem in which and with which the currents which are to be commutatedin the pulse-controlled inverter can be significantly reduced in asimple and cost-effective way.

This object is achieved according to the invention by means of agenerator/motor system having the features of patent claim 1 as well asa method for operating this generator/motor system having the featuresof claim 8.

It is advantageous in particular that by dividing the pulse-controlledinverter into two identical pulse-controlled inverters with half therated power each it becomes possible to operate the generator/motorsystem both in a star circuit and in a single phase circuit and as aresult to obtain uniform current loading of the filter over a widerange. As a result, both a peak current during starting and aconfiguration of the filter to this peak load are avoided since in thestar circuit only approximately half the conventional phase currentshave to be commutated.

This, and further objects, advantages and features of the invention,become apparent from the following description of a preferred exemplaryembodiment in conjunction with the drawing, in which:

FIG. 1 is a circuit diagram of a generator/motor system according to theinvention,

FIG. 2 is a torque/rotational speed characteristic of a conventionalgenerator/motor system and an equivalent torque/rotational speedcharacteristic of the generator/motor system according to the invention,and

FIG. 3, with FIGS. 3A and 3B, shows a conventional drive system and anassociated rotational speed/torque characteristic.

FIG. 1 shows a circuit diagram of a generator/motor system according tothe invention. The generator/motor system according to the invention hasa three phase rotational field machine DM whose individual generatorphase windings or machine phases a, b and c are connected to a first andsecond pulse-controlled inverter PWR1 and PWR2. The first and secondpulse-controlled inverters PWR1 and PWR2 are of identical design andhave the same rated power. Each pulse-controlled inverter PWR1 and PWR2is composed of six electronic branch switches S1 to S6 which are formed,for example, by MOS transistors or IGBT (Integrated Gate BipolarTransistors) and are arranged symmetrically in series in three branchpairs, and of a filter capacitor C1 and C2 which are connected inparallel with the pulse-controlled inverter. As a result of the divisioninto the first and second pulse-controlled inverters PWR1 and PWR2 it ispossible to select significantly smaller capacitors for these filtercapacitors C1 and C2, which have an advantageous effect on overall sizeand power loss.

Between the two pulse-controlled inverters PWR1 and PWR2, an electronicswitch S7, via which a positive busbar of the first pulse-controlledinverter PWR1 can be connected to the positive busbar of the secondpulse-controlled inverter PWR2, and disconnected from it, is formed inparallel with the machine phases a, b, c. This electronic switch S7 can,but does not need to, be bidirectional. A power MOS transistor with aparasitic reverse-biased diode can be used as a nonbidirectional switchfor the switch S7.

The method of operation of the generator/motor system according to theinvention will now be explained below with reference to FIG. 1.

The generator/motor system according to the invention permits twodifferent operating modes.

1. Operation with a Star Circuit

In a star circuit the branch switches S1, S2 and S3 are closed and thebranch switches S4, S5 and S6 as well as the electronic switch S7 areopen. The pulse-controlled inverter PWR1 thus forms a star point for thethree machine phases a, b and c which are represented. Involtage-impressing pulse-controlled inverters PWR1 and PWR2 in a sixpulse bridge circuit, the potential of the star point jumps between ⅓and ⅔ of the voltage of the intermediate circuit as a function of theswitched-on voltage vectors. If the switches are composed of MOSs thereverse-biased diode does not need to be activated.

Since, compared with the prior art, only half the pulse-controlledinverter, specifically the pulse-controlled inverter PWR1 is nowavailable for conducting current, the rotational field machine DMacquires more stator turns as compensation for this. The flux linkage,which determines the torque, is thus retained. As a result, thecharacteristic curve branch 1 in FIG. 2 is obtained. An equally largetorque is implemented but since only half the circuit participates inthe conversion of energy, specifically the pulse-controlled inverterPWR1, it is also the case that only approximately half the originalphase currents have to be commutated. If the same ripple of theintermediate circuit voltage is permitted, the filter expenditure isalso approximately halved.

2. Operation with a Single Phase Circuit (“Open Delta”)

Of course, owing to the approximately double number of stator turns ofthe rotational field machine DM, the modulation limit of thepulse-controlled inverter PWR1 is already reached at half the rotationalspeed in comparison with the standard solution. The star point which isformed by the pulse-controlled inverter PWR1 is then eliminated and thegenerator/motor system is operated with a single phase circuit. For thispurpose, the electronic switch 7 is closed and the pulse-controlledinverter PWR1 is actuated in such a way that each phase receives its ownhalf bridge, i.e. all the branch switches of the first and secondpulse-controlled inverter PWR1 and PWR2 are closed. By reducing theterminal voltage to approximately half, the modulation limit of thegenerator/motor system according to the invention is moved furthertoward higher rotational speeds. The same dimensioning point isimplemented. A characteristic curve branch 2 of the characteristic curveof the generator/motor system according to the invention is thenapproximately covered with that of one standard circuit.

As a result, the objective is achieved using the switchablegenerator/motor system according to the invention. By switching over themotor/generator system the current loading of the filter is homogenizedover a wide range. The peak current during starting and theconfiguration of the filter to this peak loading is thus avoided.

Switching over from one operating mode to the other is carried outaccording to the invention in a way which is optimized in terms ofefficiency. Only the maximum characteristics are shown in FIG. 2. With apartial load, a control unit which can be implemented as software moduleassumes the precise characteristic-diagram-dependent switchover point ina way which is optimized in terms of efficiency. Since the switchingover takes place without an impact, it is in theory possible to switchover as often as possible.

Furthermore, an advantage with the circuit according to the invention isthat some of the capacitor quiescent currents can be switched off usingthe switch S7.

In addition the reliability is increased since the single phase circuitpermits operation, however somewhat restricted operation, withasynchronous machines even if an electronic switch in thegenerator/motor system has a fault, for example short circuit ordisconnection). It is then always also possible to build up a rotationalfield, which is not possible with a standard bridge circuit with threephases.

In addition, the efficiency is increased since the reduction in theripple currents not only leads to a reduction in the filters overall butalso to a reduction in the filter losses.

1. An electric generator/motor system, in particular for application in mobile units, motor vehicles, ships and the like as an on-board power system generator and starter, having: a rotational field machine (DM) with three generator phase windings (a, b, c) and a pulse-controlled inverter which has a predetermined maximum power and is connected to the three generator phase windings (a, b, c) of the rotational field machine (DM), characterized in that the pulse-controlled inverter is divided into a first and a second pulse-controlled inverter (PWR1, PWR2) which are identical to one another and which have half the maximum power, the first and second pulse-controlled inverters (PWR1, PWR2) each have three branch pairs (S1, S4; S2, S5; S3, S6), each of the three branch pairs (S1, S4; S2, S5; S3, S6) is connected to an associated winding of the three generator phase windings (a, b, c) and is composed of at least two symmetrically arranged electronic branch switches (S1 to S6) which are located in series with one another in the same direction, the branch pair (S1, S4; S2, S5; S3, S6) is connected to a d.c. voltage source via the branch switches (S1 to S6), wherein the generator phase windings (a, b, c) are connected between a pole of the d.c. voltage source and the center point of the associated branch pair (S1, S4; S2, S5; S3, S6), in each case a filter capacitor (C1, C2) is connected in parallel with the branch pairs (S1, S4; S2, S5; S3, S6) of the first and second pulse-controlled inverter (PWR1, PWR2), and an electronic switch (S7) is formed by a positive busbar which connects the first pulse-controlled inverter (PWR1) and the second pulse-controlled inverter (PWR2) to a positive pole of the d.c. voltage source and via which the positive busbars of the pulse-controlled inverters (PWR1, PWR2) can be connected and disconnected from one another.
 2. The electric generator/motor system as claimed in claim 1, characterized in that the electronic switch (S7) is unidirectional.
 3. The electric generator/motor system as claimed in claim 1, characterized in that the electronic switch (S7) is a power MOS transistor with a parasitic reverse-biased diode.
 4. The electric generator/motor system as claimed in claim 1, characterized in that the electronic switch (S7) is a bidirectional switch.
 5. The electric generator/motor system as claimed in claim 1, characterized in that the branch switches (S1 to S6) are power MOS transistors with a parasitic reverse-biased diode.
 6. The electric generator/motor system as claimed in claim 1, characterized in that the rotational field machine (DM) has such an increased number of stator turns that when only one pulse-controlled inverter (PWR2) is connected into the circuit it is possible to bring about a flux linkage which corresponds to a flux linkage when the entire pulse-controlled inverter, i.e. the first and second pulse-controlled inverters (PWR1, PWR2) are connected into the circuit, without increasing the number of stator turns.
 7. The electric generator/motor system as claimed in claim 1, characterized in that furthermore a control unit is provided which under partial load implements a characteristic-diagram-dependent switchover point from a star circuit operating mode into a single phase circuit in a way which is optimized in terms of efficiency.
 8. A method for operating a generator/motor system as claimed in claim 1, characterized by the steps: operation of the generator/motor system in a star circuit by keeping closed the branch switch (S1 to S3), arranged on the side of the positive pole of the d.c. voltage source, of the first pulse-controlled inverter (PWR1) and keeping open both the branch switches (S4 to S6) which are arranged on the side of the negative pole of the d.c. voltage source and the electronic switch (S7) as well as all the branch switches of the second pulse-controlled inverter (PWR2); sensing the rotational speed of the rotational field machine (DM) and determining a characteristic-diagram-dependent switchover point; switching over the generator/motor system at the determined switchover point to operation in the single phase circuit by means of the control unit by closing the electronic switch S7 and actuating the first pulse-controlled inverter in such a way that each generator phase winding (a, b, c) receives its own H bridge, i.e. by all the branch switches of the first and second pulse-controlled inverter (PWR1, PWR2) being closed.
 9. The method as claimed in claim 8, characterized in that the switchover point is determined in a way which is optimized in terms of efficiency. 